All the credit for this mod goes to bbottomley for his very detailed PM a while back.
I just now got to doing this mod and wanted to share his original instructions, along with the variations I had to make to hardwire my TomTom GPS unit.
Addendum: The following write-up explains one method of hardwiring a GPS or other 12V cigarette adapter-powered unit. As with any other project or modification, there is more than one way to do this. Please refer to the additional discussion here later on in the thread. Bottom line: This hardwiring solution is for those who value a minimalistic, free (short of wire and solder) approach at the expense of modifying your GPS's original charger.
Originally Posted by bbottomley
First, a couple of cautions:
I assume that your GPS is USB-powered. All bets are off it uses a different source of power.
There's no guarantee that your charger will be constructed quite the same way as mine, so consider these instructions to be generic in nature.
The first step in opening it up is to unscrew the ring that surrounds the center terminal. The ring, the center terminal, and the fuse should easily fall out.
Depending on how your charger is constructed, that might be all it takes to be able to separate the two halves of the housing. Or there might be a screw. Or there might be internal plastic clips. Clips was what I encountered, and gentle prying with a small screwdriver, starting at the USB end, was all it took to snap it open. If you should break a clip in the process, I'm sure you could tape the two halves back together without any problem.
At this point you should see something like this.
(I didn't notice till after I'd put my charger back together that I had reversed the threaded ring in the picture, but no big deal.)
Here's a closeup of the part that matters.
All that's left now is to remove the positive terminal spring and the negative terminal spring strips, solder wires to the red and the black wires that were connected to them (carefully insulated of course), run the wires out through the hole where the fuse and center terminal used to be, put things back together, and run the black wire to ground and the red to an appropriate 12 volt source. Depending on the source you choose you might want to use an inline fuse somewhere along the way. Note that the charger will always draw power, even when the GPS is off, so I wouldn't recommend choosing an always-on source.
Check for sharp edges on the ring before reinstalling it. You wouldn't want it to cut into the insulation. Wire gauge isn't critical here. Anything in the 18-22 gauge range would be fine.
I got my charger from DealExtreme in Hong Kong. See link in my Sources Thread
. You'll have to find an appropriate USB cable to match on both ends.
Here's my version:
I had originally planned to do this mod just as above. Use a cheapo USB cigarette lighter adapter, and run my own appropriate USB cable (male type A to male 90* mini USB in my case.) However, a little research turned up that the TomTom XL's charger cable also acts as a traffic antenna/receiver for the unit. Without that specific charger, no joy on traffic updates, which I especially like on my TomTom. This was a feature I had to keep, so I decided I would instead take the risk and cannibalize the costly TomTom charger. As you can see in this picture, the TomTom unit recognizes it's included charger as a "RDS-TMC" receiver. With a plain old USB cable, you don't get this notification.
So the first step was to take apart the cigarette lighter adapter piece, just as bbottomley described. Almost all adapters should disassemble this way, since I would guess the fuse access is industry-specific. Here's my charger taken apart. If you're picky like me, discharge the capacitors once you've gotten this far. Hold on to that fuse, you'll need it later.
You'll notice that my charger is slightly different than the one above in that the circuit board has 2 grounds. Both the metal spring strips are connected to the circuit as a ground, instead of just one. All this means is that I had to solder 2 grounds wires in, and tie them together. Pictured below are my soldered and carefully insulated connections coming off the circuit board.
And here's the body of the piece clipped back together with the new connections running out. I later taped the body together with electrical tape for extra security.
Here are the two vital pieces for the rest of the wiring. On the left is an in-line fuse holder into which I replaced the fuse from the TomTom's original charger. This may be overkill as the fuses are likely of the same specs, but I figured it couldn't hurt. On the right is a mini-fuse tap I got from Advance Auto Parts. More on that later.
Here's the wiring all done. The red +12V comes out of the charger, through the in-line fuse, and to the mini-fuse tap. The ground comes straight out and terminates as a ring connector.
Now for the installation. Now would be a good time to disconnect the negative terminal of your battery. The fuse tap lets you steal power from an existing circuit, but essentially creates a new fused circuit off of it. All you do is replace the existing fuse in the fuse box with the fuse tap. The lower (pictured) fuse is the fuse for the original circuit. Here I used the same 15A fuse I removed from the "MIRROR/CIGAR" circuit. The upper (pictured) fuse with the corresponding red wire is the fuse for the new circuit being created. I just used a 10A as it was on-hand, and is really overkill since I already wired in an in-line fuse.
Now install the fuse tap unit into the fuse slot you originally removed from, and wire the ground wire to a solid nearby ground. I used the bracket for the hood release handle.
Once you've verified everything is working, continue by routing the wires however/wherever you like to get to the device you will be powering. As a temporary fix, I ran my wire up to the left of the steering column cover up to my custom TomTom mount which you can view here
. Eventually, I'll sneak the wire behind the cluster and out the trim corner like Bobby (2.5x_sleeper) did with his Scangauge II wiring. More about that here
. Zip tie up the extra wiring and you're done!
And the best part... The ashtray cover can stay neatly closed and your cigarette lighter/12V outlet is available for another device.